The present invention generally relates to semiconductor devices and more particularly to a fabrication process of a semiconductor device including a metallization process.
Metallization process is a process indispensable for fabricating a semiconductor device. For example, a metallization process is used when forming a conductor layer of Al or an Al-alloy on a substrate. Generally, a metallization process is carried out by a sputtering process.
When fabricating a Si semiconductor device, it is commonly practiced to form a diffusion barrier of TiW, TiC, TiN, and the like, between an interconnection pattern, which may be formed of Al or an Al-alloy, and a bonding region defined on a Si substrate, prior to the metallization process of a conductor layer from which various interconnection patterns are formed. Such diffusion barriers are used for preventing the formation of alloy spikes that otherwise would be formed as a result of diffusion of Al from the interconnection pattern into the Si substrate across a thin diffusion region formed on the Si substrate.
Generally, a TiN diffusion barrier is formed by a reactive sputtering process in which the sputtering of Ti is carried out in a nitrogen plasma environment while using a high purity Ti target. While it is possible to use a TiN target for the sputtering of a TiN film, use of a TiN target tends to cause a problem of excessive deposition thickness for the TiN film thus formed on the substrate. When the thickness of the TiN film is excessive, there appears a tendency that the TiN film comes off easily from the substrate. Because of this reason, a TiN target is not used in the formation of TiN diffusion barriers.
It should be noted that a diffusion barrier, being used in a contact region in which an interconnection pattern is connected electrically to a semiconductor device, is required to have a low resistance as low as possible, in addition to a dense film structure or texture, which is a requisite of an effective diffusion barrier.
A TiN film is also used for a glue layer of a W layer or an anti-reflection film of an Al interconnection pattern. When a TiN film is used for a glue layer of a W layer, it is desired that the TiN film experiences little corrosion when exposed to WF.sub.6, which is generally used for a CVD gaseous source of a W layer. In order to meet for such a demand, the TiN film is again required to have a high density texture.
On the other hand, in the conventional reactive ion etching process that uses a Ti target, there has been a problem in that the condition in which a TiN film is formed stably and reproducibly with high density and/or low resistance is limited substantially. Thus, it has been difficult to form a high density, low resistance TiN film suitable for a diffusion barrier efficiently and hence with a high throughput of production.
When forming a TiN diffusion barrier by a sputtering process, it is generally practiced to carry out a dummy sputtering process first on a dummy substrate for cleaning the surface of the Ti target to be used for the sputtering, such that any impurities on the surface are removed during such a dummy sputtering process. However, the phenomenon of reactive sputtering is not fully understood yet, and there has been a problem in that the use of such a cleaned Ti target tends to narrow the range of the sputtering condition in which a satisfactory TiN film is obtained reliably and reproducibly.
More specifically, it is known that no TiN film is formed in a reactive sputtering process conducted in a plasma of a gas mixture of Ar and N.sub.2, when the proportion of Ar in the gas mixture is large and the proportion of N.sub.2 in the gas mixture is small. In this respect, it is known that a TiN film is readily obtained when the proportion of N.sub.2 in the plasma gas mixture is increased. However, the TiN film thus obtained tends to show a low density texture and correspondingly high resistance not suitable for a diffusion barrier. Thus, the optimum condition for obtaining a TiN film suitable for a diffusion barrier is substantially limited with respect to the gas mixture composition, sputtering power, and the like.
It is well known that TiN forms a non-stoichiometric compound represented by TiN.sub.x. Thus, a TiN film having a coarse and correspondingly low density texture is tend to be obtained when the N.sub.2 proportion in the plasma gas mixture is large. The coarse TiN film thus formed generally includes coarse TiN crystals aligned in a &lt;111&gt;-direction. When the proportion of N.sub.2 in the plasma gas mixture is reduced, on the other hand, a TiN film having a dense texture can be obtained. The dense TiN film thus formed generally includes fine and uniform TiN crystals aligned in a &lt;200&gt;-direction. The dense TiN film thus obtained has a further advantageous feature in that the surface of the TiN film is smooth and flat. However, reproducible or reliable formation of such a dense TiN film is substantially difficult, in view of the fact that the sputtering has to be conducted under a narrow condition in which the N.sub.2 proportion is set small.
It is also known that a high density TiN film suitable for a diffusion barrier may be obtained by a plasma sputtering process of TiN conducted under a large plasma power. However, such a sputtering of TiN conducted under a large plasma power tends to require an increased N.sub.2 content in the plasma gas mixture. Thus, when the N.sub.2 content in the plasma gas mixture is excessive, the obtained TiN film tends to show a low density and a large electric resistance.